The objective of this proposal is the serological and functional definition of genetically specified embryonic cell surface antigens important in both normal and malignant embryonic differentiation. Two biological tools will be utilized; first, embryonal tumor cells, primarily teratoma lines, which will be studied for surface antigens indicative of their particular state of differentiation. Second, a series of allelic mutations at the T-locus of the mouse which are known to interfere with specific and different switchpoints in embryonic development. These mutations specify surface antigens on sperm and are presumed to also specify antigens on embryonic cells. As several of these mutations are embryonic lethals, their surface antigens are thought to be functional in development, primarily in cell-cell recognition. These t-mutants antigens will be sought and defined on embryonic cells and developmentally blocked embryonal tumor cells.